Much of this info courtesy of the Premier Guitar article by Peter Stroud (Sheryl Crow, 65 Amps)

 

Here I address an issue that every guitarist will have to deal with once they step out of the house and into a club.

 

It goes like this: Ever wondered why your tube guitar amp doesn't sound nearly as good as it does (usually) at home? Haven’t you experienced those times when the sound is fuzzy, anemic or just plain lifeless? You might even be hearing all sorts of loud buzzes and hums that you attribute to "the stage lights" or something, particularly with your single coil pickups.

It's definitely all about poor AC power conditions, and sometimes grounding and RF (radio frequency) interference. Most often the problem is low voltage that can't keep up with the demand for current exacted by your guitar amp and the rest of the stage backline, lighting and PA. Power issues are the most common headache on a club tour, and hours are spent trying to work around it.

Let me tell you, it's the same scenario on the "big" stages, including theaters, sheds and arenas. Every day is different, too. Sometimes the voltage levels are great, other days the voltage is low, so that you get noise and lifeless tone from the amps. Even when the power is being supplied and conditioned on stage from a single source, we still have to rely on the voltage supply offered by the venue. If the voltage is low, your tone is most likely going to suffer, badly.

Some amplifiers—particularly tube guitar amps-- are more sensitive to low voltage than others are. If you're going for a cleaner sound, you'll probably hear the dirty, flat, lifeless tone first. Then again, if you're high gain you'll hear the hums and buzzes quickly. 

Amp techs and experienced players who know better have their guitar amp running on a Variable Autotransformer, most commonly known as "variac" ("Variac" is actually the brand name of the General Radio Company's variable autotransformer). Many consider it a cool trick since it’s possible to goose the voltage a tad in order to get a little more oomph out of the amps.

 

The idea of using a variac is to keep the voltage constant from night to night, show to show.

Every night guitar amplifiers will sound clear, full and punchy, the rack is quiet (though some effect pedals will hum and buzz if they don't see the current they need) and single-coil pickups don't hum nearly as much as they used to. It makes a world of difference.

So, great...I'm sending you out to get a variac. However, you'll find it isn’t that easy. So, be careful and choose wisely. Do a Google or eBay search on variacs, and thousands of choices will pop up, in all types and prices.

Here's any easy reference of what to look for. You can buy one from any of these three guys:

 

1. General Radio

2. Staco

3. Superior

What commonly works out well for most is a "single-phase portable transformer with an input of 120VAC and output range of 0-140 VAC / 7.5 or 10amps (a built-in digital voltmeter is a plus but adds to the cost).”

The important thing to note when buying a variac is its current rating. But you might ask, is that 2A (amps), 5A or 10A? The math is easy, but it’s important. For instance, how many guitar amplifiers and effects will you be plugging in? A quick guide is to check the VAC fuse value of your guitar amplifier(s.) If it’s a 3A fuse rating, then most likely your amplifier draws around two amps of current when pushed. If you're running two guitar amplifiers at the same time, you might be pushing four to six amps of current at peak points. Pedals and pedalboards usually draw very low amounts of current. You might get by just fine with a variac rated at 5A current output. So, 7.5A or 10A current output should cover most any need. Go beyond than that and the physical size of the variac becomes an issue, as well as the price.

WARNING! YOU HAVE TO BE CAREFUL USING VARIACS! If you accidentally hit the knob and boost the voltage up toward 140VAC, you stand the chance of sending your rig to Mars… or at least to the repair shop with a lot of burned-out components and tubes. Hopefully the VAC fuse will blow instead. It’s also possible to cause damage if you're running it at way too low a voltage. Again, the intention here is to have your rig running at its proper voltage!

The answer is to use your ears. Most new guitar amplifiers like to see 117VAC to 120VAC. Most modern amps are wired for 120VAC power. Most homes these days show 119-121VAC coming out of the wall socket. Most stages might get up to 118VAC on a good day but will otherwise be lower, which can adversely affect your sound.

WATCH THAT VOLTMETER! If your new variac doesn't have one built in, check the AC output using a voltmeter. HOWEVER, IF YOU DON'T KNOW HOW TO CHECK THE AC VOLTAGE, THEN YOU SHOULDN'T BE DOING THIS IN THE FIRST PLACE.

Still with me, good.

 

With a variac, I suggest the following power-up sequence: